I think it is the best forum thread for the question about databases. I know that DH provides MySQL databases in all types of account and now I'm using it. Sometimes MySQL is not good enough (e.g. complex queries) this is why I'm dreaming about something like PostgresSQL server as one of the options of my account. As I know it is free and can be used without any restrictions.
Are you planning to create some PSQL servers? Is it possible to use them?